mingw: poll.h is no longer in sys/

Earlier we moved this header file in the code but forgot to
update the Makefile that refers to it.
Signed-off-by: NErik Faye-Lund <kusmabite@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Makefile

@@ -515,7 +515,7 @@ LIB_H += compat/mingw.h
 LIB_H += compat/obstack.h
 LIB_H += compat/win32/pthread.h
 LIB_H += compat/win32/syslog.h
-LIB_H += compat/win32/sys/poll.h
+LIB_H += compat/win32/poll.h
 LIB_H += compat/win32/dirent.h
 LIB_H += connected.h
 LIB_H += csum-file.h